A built-in line testing mechanism is desired in many communications systems, since it allows the link to be tested remotely without sending service personnel to the site. Such mechanisms are often available in e.g. Ethernet transceivers and xDSL transceivers. For example, single- and double-ended line testing measurements and post-processing for xDSL are described in the ITU-T standard G.996.2, Single-Ended Line Testing (SELT).
Single-Ended Line Testing (SELT) is a useful tool for troubleshooting non-working communication links or links with degraded performance and for locating cable faults. SELT is also useful for e.g. cable installation verification and performance estimation/prediction. For full-duplex, echo-cancelled systems like 1000BASE-T and 10GBASE-T, it is possible to transmit and receive at the same time and at the same frequency. This enables both echo and transfer function measurements. However, for communication systems applying FDD, it is in general not possible to transmit and receive at the same frequency unless some switching mechanism is added in order to bypass either the transmit or receive filters or both. If switching needs to be performed in analog domain, which is typically the case when analog duplex filters are involved, the switches add cost and complexity and may also decrease system performance since communication signals must pass through the switches during normal operation.